4.2 DCIV Method

Another experimental method for the investigation of the interface quality is the direct-current current-voltage (DCIV) method. It can be used to determine both the interface trap density and the amount of fixed oxide charges .

Figure 4.10: Configuration of the DCIV measurement. The source and drain to substrate diodes are slightly forward biased. As the gate to substrate voltage is swept, a current at the substrate contact can be measured.

The experimental set-up is illustrated in Figure 4.10. The source and drain to substrate diodes are slightly forward biased, typically with around $\ensuremath{V_\textrm{e}}= \ensuremath{V_\textrm{s}}= \ensuremath{V_\textrm{d}}\approx 0.33$V. The gate bias is swept from inversion to slight accumulation. During this sweep the substrate current is measured. The substrate current originates from recombination of carriers at the interface.

Figure 4.11: DCIV currents after different stress times. With increasing interface degradation the maximum increases. The shift of the peak position to lower voltage indicates fixed oxide or interface charges. Data are from Zhu et al. [45].

For each degradation level there is a clear peak of at a certain $\ensuremath{V_\textrm{g}} = \ensuremath{V_\textrm{peak}}$, as shown in Figure 4.11. The peak height above the base line, is approximately proportional to the effective interface trap density as [46,47]

$$\ensuremath{\Delta I_\textrm{DCIV}}= \frac{1}{2}\ensuremath... ...e}}\vert}{2\ensuremath{\textrm{k$_\textrm{B}$}}T} \right) .$$ (4.7)

Here, is the elementary charge, the intrinsic carrier concentration, $\sigma$ the geometric mean of the capture cross sections for electrons and holes, equal to $\sqrt{\sigma_\mathrm{e}\sigma_\mathrm{h}}$, the thermal velocity, $A$ the effective gate area, the forward bias applied to the source-substrate and drain-substrate junctions, Boltzmann's constant, and $T$ the temperature.

To determine , the oxide charge density, the peak position of the DCIV curve can be used. As described in Section 2.2.1, the peak position is directly proportional to the amount of oxide charges and can be calculated using Equation 2.31.